The present invention relates to a fluid-displacement machine, such as a pump or motor, in general, and more particularly to a radial piston machine.
Fluid-displacement machines of the above-mentioned type are already known in a variety of constructions. Such fluid-displacement machines include stator and rotor components, a plurality of cylinder-receiving passages in the rotor, a plurality of pistons each accommodated in one of the passages and having a cam follower portion, at least one cam track which extends circumferentially about the above-mentioned axis, and biasing means which urges the pistons toward the cam track so that the cam follower portions of the pistons follow the configuration of the latter. In addition thereto, the conventional machines also include at least one control member, such as a control sleeve, which is interposed between the working chambers of the respective passages and the respective low-pressure and high-pressure conduits of the machine and controls the communication of the respective working chambers with the respective conduits.
One conventional machine of this type which is disclosed in the German patent DT-PS 2,209,996 is an axial piston machine the rotor of which accommodates two sets of pistons for reciprocation in the respective passages. This conventional machine includes two cam tracks, each arranged at one axial end of the machine and each being associated with one of the sets of the pistons. One of the above-mentioned cam tracks is mounted on the machine for angular movement about the axis thereof by means of a worm transmission which can be actuated from the exterior of the machine. A pinion is mounted on one end face of the rotatable control sleeve between the outer periphery of the rotor and the inner periphery of the housing of the machine, the pinion meshing with a gear annulus of the housing and also with a gear annulus of the angularly movable cam track so that, when the cam track is angularly moved relative to the housing, the control sleeve is angularly displaced by one half of the angle of movement of the movable cam track. In this machine, always two axially opposite pistons of the two sets of pistons are accommodated in a common passage.
The cam tracks of the above-mentioned conventional axial piston machine are provided on respective disks, and each of the cam tracks has a plurality of, such as nine, raised portions. Thus, during each of the rotations of the rotor, each of the cam follower portions, in cooperation with the associated biasing spring or the like, will reciprocate the associated piston a number of times which corresponds to the number of the raised portions of the respective cam track. The cam tracks of the disks are so arranged that, when the machine is being used as a motor, the sum total of the driving torques is maintained constant. On the other hand, when the machine is being used as a pump, the configurations of the cam tracks are such that the output pressure of the pump remains constant. As a result of the positively simultaneous angular displacement of the cam disk which carries the cam track and the control sleeve, via the interposed pinion, it is possible to steplessly change the amount of the working fluid. Furthermore, it is possible to reverse the direction of flow of the working fluid without any need for providing any additional valves or the like.
However, the above-mentioned machine is also disadvantageous in some respects. First of all, the reaction torque must be applied to the rotatable cam disk. Thus, when the cam disk is to be angularly moved, a portion of the reaction torque must be overcome. Thus, the angular movement or adjustment requires a considerably high expenditure of force. Correspondingly to this, there results the disadvantageous requirement for the provision of a reduction gear train which then requires a correspondingly high expenditure in terms of labor and materials.
In addition thereto, the above-explained axial piston machine renders it possible to achieve a pulsation-free operation, at the very best, only for a single optimized adjustment point. On the other hand, when this optimum point is departed from as a result of the angular adjustment of the cam disk or of the control sleeve, a pulsation-free input or output volume of the working fluid is no longer obtained. Furthermore, the conventional machine of this type does not possess a uniform input or output torque at constant loading throughout the adjustment range thereof.